2. Field of the Invention
This invention relates in general to compositions and methods for decomposing drilling muds. In one aspect, the invention relates to compositions and methods for cleaning up well site drilling mud pits. In another aspect, the invention relates to compositions and methods for removing residual drilling muds from subterranean formations and/or wellbores.
2. Description of the Prior Art
Drilling mud is an essential part of most oil and gas drilling operations. The mud is continuously circulated down through the drill pipe, out through the drill bit and back to the surface through the annulus between the drill pipe and the wall of the bore or casing. The mud serves to lubricate and cool the drill bit as the bit breaks up rock at the bottom of the hole and carries the cuttings to the surface. The mud also helps control pressures that exist in formations penetrated by the drill bit, stabilizes the hole and serves as a valuable source of downhole information.
When drilling mud returns to the surface, it is typically conducted over a shale shaker where relatively large drill cuttings are removed. These large drill cuttings are placed in a shale pit. The mud is then conducted into a settling pit where a portion of finely ground rock (silt) in the mud is deposited. After possibly being run through one or more other solids removal or "working" pits, the mud is pumped back into the well through a suction pit. Fresh chemicals are typically added to the mud while it is in the suction pit. Water can be added to the mud both at the shale shaker and in the suction pit.
On most rigs, a large volume of drilling mud and other material is discarded into one or more "reserve" pits. The mud and material in the reserve pit(s) are kept for use if needed to prevent lost circulation or fight a blow out, etc. During the drilling process, solid material that builds up in the working pits is cleaned out by jetting the pits. Because water is continuously added to the mud, the volume of mud on hand increases and becomes excessive. The excess mud is often used to clean out the working pits. The solids-laden mud from the cleaning process is typically pumped into the reserve pit(s). On some rigs, rig wash water and other run-off material is also deposited in the reserve pit(s). Depending on the location and type of operation, drilling mud remaining at completion of the well is also pumped into the reserve pit(s). The primary solids in a reserve pit are drilled solids. In addition, a reserve pit will typically contain a significant portion of solids that are part of the original mud itself, particularly if the mud is a weighted mud. At the conclusion of the drilling operation, the agglomeration of mud and other materials in the reserve pit(s) must be disposed of so that the well site can be restored to a proper condition.
Most drilling muds typically contain one or more natural and/or synthetic polymeric additives including polymeric additives that increase the rheological properties (e.g., plastic viscosity, yield point value, gel strength) of the drilling mud (hereinafter called "polymeric organic viscosifiers") and polymeric additives that do not increase the rheological properties of the mud such as thinners and flocculents. Polymeric organic viscosifiers are employed for many purposes. For example, polymeric organic viscosifiers are employed to impart sufficient carrying capacity and thixotropy to the mud to enable the mud to transport the cuttings up to the surface and to prevent the cuttings from settling out of the mud when circulation is interrupted. Polymeric organic viscosifiers are also employed to limit fluid loss to the formation and to prevent an excessive filter cake from forming on the wall of the bore. Most of the polymeric organic viscosifiers employed in drilling mud are highly resistant to biodegration. This extends the utility of the additives for the useful life of the mud.
Unfortunately, the resistance of drilling mud polymeric organic viscosifiers to biodegration causes many problems. Polymeric organic viscosifiers make it very expensive and cumbersome to dispose of the agglomeration of drilling mud and other materials in reserve pit(s) in an environmentally safe manner. Typically, between 5,000 and 50,000 barrels of solids-laden mud and other materials have accumulated by the end of a drilling operation. In some drilling operations, as much as 100,000 barrels of mud may remain after the well is completed. The polymeric organic viscosifiers inhibit flocculation and prevent accumulated solids from settling out of the mud. As a result, special solids separation equipment such as decanter centrifuges, cyclone separators, fine screen "rumba" shakers and the like is usually required to separate the solids from the water phase of the mud. This type of separation equipment typically involves dilution of the mud creating even more water that must be disposed.
In addition, polymeric organic viscosifiers remaining in the well tend to interfere with other phases of drilling and completion operations such as cementing the casing to the wall of the bore as well as optimum oil and gas production after the drilling operations are complete. As the bore is initially drilled, fluid from the mud tends to seep into the surrounding earth forming a filter cake on the wall of the bore. This filter cake can prevent casing cement from properly bonding to the wall of the bore. The trajectory of a wellbore is generally tortuous. The wall of the bore often has various ledges and cavities therein which contain thixotropic drilling mud that comes into contact therewith. The drilling mud in contact with the bore wall is quiescent while the casing is lowered into the bore and tends to gel. When circulation is resumed, the fluid pumped through the casing and up through the annulus between the casing and the bore wall makes paths or channels or even bypasses the "gelled" mud contained by the ledges and cavities.
Cement pumped through the casing and up through the annulus to cement the casing to the bore wall flows through the paths or channels in the mud leaving large pockets of mud between the casing and the bore wall. These pockets can ultimately result in fluid communication with formation zones that the cement is supposed to isolate. In an attempt to solve this problem, special fluids are often circulated through the annulus between the casing and the wall of the bore before the casing is cemented to remove mud remaining therein. Unfortunately, this procedure, often referred to as a "spacer" flush, is inadequate in many applications. Conventional flushing fluids are not always capable of sufficiently decreasing the gel strength, viscosity and other rheological properties of the mud caused by polymeric organic viscosifiers therein. As a result, the mud cannot be flushed out of the well. Instead, expensive squeeze cementing operations are carried out to fill in the gaps in the cement caused by the mud.
Furthermore, the hydrostatic head pressure created by the mud may fracture the formation resulting in lost circulation, i.e., the flow of whole mud into portions of formations penetrated by the drill bit adjacent the wall of the bore. Residual mud remaining around the well face in a producing formation can impede the flow of oil and/or gas to the wellbore. Polymeric organic viscosifiers in the mud tend to tie up other mud components rendering residual drilling mud around the well face nondispersible by water and other fluids and incapable of being removed by conventional flushing techniques.